Studies of allosteric structure change and of dynamic "breathing" behavior are continuing using hemoglobin and hydrogen exchange methods. The possibility that some structural breathing reaction is required for O2 to penetrate into the heme binding site of hemoglobin will be tested by measuring O2 binding rates as O2 concentration is increased to high levels. The identity of allosterically responsive segments of hemoglobin will be tested by trying to develop appropriate fragmentation and separation methods. The responsive segments can be labeled with tritium at the exchangeable peptide NH by difference hydrogen exchange methods. The effort will be to start with samples labeled specifically at one responsive segment at a time, and then to fragment the moleculer and find which fragment carries the tritium. BIBLIOGRAPHIC REFERENCE: Comparison of bacterial and animal rhodopsins by hydrogen exchange studies. J.J. Englander and S.W. Englander; Nature 265, 657 (1977).